for starters Luke, you're not dealing with 'wee' and 'poo', the liquid in the bin is moisture from the foods and water you add to the bin .... the castings are the same material you add, bacterially decomposed to a condition that allows the worms to "slurp" off the slimey, bacteria rich surface, extract some nutrition from those microbes, and pass it on thru to be worked over again and again by the worm colony

the reason some materials disappear faster is they're materials that become microbially active faster - the higher sugar and water content of the melon result in rapid fermentation by yeast, the apple has a higher fiber [pectin] content, so needs more bacteria than yeast, a slower route .... if you chop all inputs up well, the greater surface area that results will become microbially active faster

but don't worry about the quantity of worms, if you provide a decent environment for them, they'll do fine, what's important is to not allow the bedding to get too dry [worms breathe via the oxygen in the moisture on their skins] and keep a moderate temperature - the only problem with too much food at once is the possibility of smells and other decomposer critters that you want to avoid, like mites and flies - try a thick layer of wet shredded paper on top as well as a cover on the bin

Bill